Sheet detection control for sheet-handling apparatus,particulary a printing press

ABSTRACT

Control for sensing the absence of a sheet on a cylinder of a printing press. The cylinder carries a pair of coils which during certain portions of its rotation pass in close proximity to two interconnected stationary coils, one of which is connected to a DC power supply. If a sheet is absent at this time, the grippers on the cylinder ground one of the cylinder-mounted coils to cause a current to be induced in the other stationary coil to turn on an SCR and thereby cause a control relay to be energized.

United States Patent William H. Lee

Chicago, 1.;

Alan W. Richards, Lyndhurst, Ohio 792,093

Jan. 17, 1969 July 27, l 971 Harris-lntertype Corporation Cleveland, Ohio Inventors Appl. No. Filed Patented Assignee SHEET DETECTION CONTROL FOR SHEET- HANDLING APPARATUS, PARTICULARY A PRINTING PRESS l3Claims,3 Drawing Figs.

U.S. Cl 271/57, 101/247 Int. Cl B65h 7/04 FieldoiSearch ..27l/57,53; 101/185, 247

References Cited UNITED STATES PATENTS 3,l91,530 6/1965 Fath et al.

Primary Examiner-Joseph Wegbreit Attorney-Yount, Flynn & Tarolli ABSTRACT: Control for sensing the absence of a sheet on a cylinder of a printing press. The cylinder carries a pair of coils which during certain portions of its rotation pass in close proximity to two interconnected stationary coils, one of which is connected to a DC power supply. If a sheet is absent at this time, the grippers on the cylinder ground one of the cylindermounted coils to cause a current to be induced in the other stationary coil to turn on an SCR and thereby cause a control relay to be energized.

SHEET DETECTION CONTROL FOR SHEET-HANDLING APPARATUS, PARTICULARY A PRINTING PRESS The present invention relates to a control for a sheet hanclling apparatus, such as a printing press, which operates in response to the absence or misalignment of a sheet during any cycle of operation of the apparatus.

Printing presses have been provided with sheet detectors which operate to detect the presence or absence of a properly positioned sheet on a cylinder of the press and to perform a control operation depending upon whether or not the sheet is present in proper position or is absent or improperly positioned at a given time. One such press is shown and described in Harrold et al. U.S. Pat. No. 2,578,700, issued Dec. 18, 1951. In this patent, the printing unit is thrown off if a sheet is not present in grippers on the impression cylinder at a predetermined point in the rotation of the impression cylinder. i

An important object of the present invention is to provide a novel and improved control for a sheet-handling apparatus, particularly a printing press, having a sheet detection arrangement of improved reliability and much greater simplicity than prior arrangements for this same purpose.

Another object of this invention is to provide such a control which operates from a relatively low voltage DC power supply.

Another object of this invention is to provide such a control which includes a pair of coils on a cylinder of the press which during certain portions of the cylinder's rotation cooperate with two interconnected stationary coils to provide a control signal in the event that sheet grippers on this cylinder sense the absence of a sheet.

Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment thereof, which is illustrated schematically in the accompanying drawing in which:

FIG. 1 illustrates schematically part of a printing press embodying the control of the present invention;

FIG. 2 is a schematic electrical circuit diagram of the present control; and

FIG. 3 is a elevational view showing the sheet grippers on the transfer cylinder in the FIG. 1 press.

Referring to the drawings, FIG. 1 diagrammatically illustrates the first two printing units A and B of a multiunit lithographic printing press. Each of the printing units of the press includes a plate cylinder 10, a blanket cylinder 11, and a impression cylinder 13. The printing press also includes a feeder 16 adapted to feed sheets one at a time in lapped relationship down a feedboard 17 to feed cylinder 20 which in turn conveys the sheets to an advance cylinder 21 for carrying the sheets to the first impression cylinder 13. The sheets are carried into the printing nip between the impression cylinder 13 and blanket cylinder 11 of the printing unit A and after being printed are transferred to the printing unit B by a double-sized transfer cylinder 24 which receives the sheet from the impression cylinder 13 of the printing unit A and transfers it to the impression cylinder 13 of the printing unit B. The transfer cylinder 24 is twice the size of the impression cylinders 13 and rotates one-half a revolution for each revolution of the impression cylinders 13.

Each of the cylinders which handles a sheet has sheet grippers thereon which grip the leading edge of the sheet and carry the sheet with the cylinder. Only sheet grippers for the feed cylinder 20 and for the transfer cylinder 24 are shown, the latter having two such sets of grippers because of its double size. The sheet grippers on the feed cylinder 20 have been given the reference numeral 26 while the grippers on the transfer cylinder 24 have been given reference numerals 27 and 28'-respectively. in the preferred and illustrated embodiment of the present invention, the grippers are adapted to sense the presence or the absence of the sheet between cooperating gripping members and to perform a control function in the event that a sheet is absent between two such gripping members. i

The grippers 27 on the transfer cylinder 24 may comprise, as shown in FIG. 3, a plurality of gripper fingers 27a, 27b, 271:...27h, 27:, 27] of electrically conductive material, which are spaced apart in succession across this cylinder along a line extending parallel to its axis of rotation. These gripper fingers are adapted to cooperate with respective gripper posts 29a, 29b...29h, also of electrically conductive material, to grip the leading edge of the sheet. The gripper fingers 27 are all grounded to the transfer cylinder. The gripper posts 29 are all insulated electrically from the transfer cylinder.

In like manner, at the opposite side of the transfer cylinder the individual gripper fingers 28 are spaced apart in succession axially along the transfer cylinder, and at each axial location of a gripper finger a cooperating gripper post 30 is mounted on the transfer cylinder. The gripper fingers 28 are all grounded to the transfer cylinder, while the gripper posts 30 are all electrically insulated from it.

The gripper fingers 27 and the gripper fingers 28 are mounted on respective pivot rods which are turned at predetermined points in the rotation of the transfer cylinder to move these fingers to closed or open positions, for either gripping a sheet or releasing it, in a known manner.

All of the gripper posts 29, which are positioned for engagement individually by the respective gripper fingers 27, and all of the gripper posts 30, which are positioned for engagement individually by the respective gripper fingers 28, are shown as connected to one end of a first movable coil 31, which is suitably mounted on or connected to the transfer cylinder 24 to revolve in unison with it. However, it is to be understood that the contacts of a selector switch (not shown) may be interposed between the gripper fingers and coil 31 such that any selected group of the gn'pper fingers may be disabled from operating the present control, depending upon the width of the sheets being handled. That is, for narrow sheets the outermost gripper fingers will be disabled because the sheet, even if present and properly positioned, will not be engaged by them.

The opposite end of the first movable coil 31 is directly connected to one end of a second movable coil 32 on the transfer cylinder, the opposite end of coil 32 being grounded on the transfer cylinder. The second movable coil 32 is suitably mounted on or connected to the transfer cylinder diametrically opposite the first movable coil 31. Preferably, each coil 31 and 32 is wound around a respective core of readily magnetizable material which is mounted on, but insulated from, the transfer cylinder 24.

With this arrangement, whenever any one of nondisabled gripper fingers 27 touches the corresponding gripper post 29 or any one of the nondisabled gripper fingers 28 touches the corresponding gripper post 30, the end of the first coil 31 on the transfer cylinder to which the gripper posts 29 and 30 are connected will be grounded.

A first stationary coil 34 is fixedly positioned adjacent one side of the transfer cylinder so that, as the transfer cylinder rotates, the coils 31 and 32 thereon will pass successively in close proximity to coil 34. When passing in close proximity to coil 34, the transfer cylinder coil 31 or 32 will move through or cut the magnetic flux produced by the stationary coil 34. A second stationary coil 33 is fixedly mounted adjacent the opposite side of the transfer cylinder so as to be in closely spaced, inductively coupled relationship to either coil 31 or 32 on the transfer cylinder as either moves past coil 33. Preferably, each stationary coil is wound around a respective core of readily magnetizable material. 7

Preferably, also, the cylinder-mounted coils 31, 32 have a greater number of turns than the stationary coils 33, 34.

The positioning of the coils 31 and 32 on the transfer cylinder with respect to the two sets of grippers thereon and the positions of the stationary coils 33 and 34 are such that, during the portion of each rotation of the transfer cylinder while the gripper fingers 28 are closed and the gripper fingers 27 are open, as indicated in FIG. 1, for a brief time the first coil 31 on the transfer cylinder will be in close proximity and inductively coupled to the second stationary coil 33 while moving past it, and at the same time the second coil 32 on the transfer cylinder will be in close proximity and inductively coupled to the first stationary coil 34 while moving past it. Then, after about another half turn of the transfer cylinder, at which time the gripper fingers 28 will be open and the gripper fingers 27 will be closed, for a brief time the first coil 31 on the transfer cylinder will be in close proximity and inductively coupled to the first stationary coil 34 while moving past it, and at the same time the second coil 32 on the transfer cylinder will be in close proximity and inductively coupled to the second stationary coil 33 while moving past it. In practice, the airgaps between the sets of movable and stationary coils when they are inductively coupled to each other, as described, may be about three thirty-seconds to three-sixteenths inch, for example.

FIG. 2 illustrates the present control circuit during the time when the coils 31 and 32 on the transfer cylinder are moving past the stationary coils 33 and 34, respectively, the gripper fingers 27 are open, and the gripper fingers 28 are closed.

In the FIG. 2 control circuit, the first stationary coil 34 is connected in series with a resistor 35 and a normally closed switch S across a DC power supply 36 which may be on the order of 24 volts, for example. Another resistor 37 is connected across the power supply 36 and switch S in parallel with the series-connected resistor 35 and coil 34. As long as switch S remains closed, coil 34 conducts direct current and produces electromagnetic flux.

A control element in the form of a relay coil R and a semiconductor switch device in the form of a silicon-controlled rectifier 38 are connected in series with each other and switch S across the power supply 36. The second stationary coil 33 is connected between the gate electrode of SCR 38 and the ground terminal of power supply 36. A resistor 39 is connected in parallel with coil 33.

In the operation of this circuit, during those portions of the rotation of the transfer cylinder 24 when its coils 31, 32 are not passing in close proximity to the stationary coils 33, 34, there will be substantially no current flowing in the coils 31 and 32 on the transfer cylinder, the SCR 38 will be off and relay coil R will be deenergized.

When the transfer cylinder reaches a rotational position in which its coil 31 is moving past the stationary coil 33 and its coil 32 is moving past the stationary coil 34, the SCR 38 will remain off and relay coil R will remain deenergized only if all of the gripper fingers 28 which are then connected to coil 31 are separated from the respective gripper posts 30 by a properly positioned sheet engaged between them. In that event, the coil 31 on the transfer cylinder will remain open circuited andsubstantially no current will flow in coils 31 and 32 despite the fact that coil 32 is cutting, or moving through, the electromagnetic flux produced by stationary coil 34.

However, a voltage will be induced across coil 32 as it cuts the electromagnetic flux of coil 34. if any one or several of the gripper fingers 28 is then making direct contact with the corresponding gripper posts 30, thereby grounding the upper end of coil 31 in FIGS. 1 and 2, the voltage induced across coil 32 will be impressed on coil 31. The resulting current in coil 31 causes it to produce electromagnetic flux which is cut by the stationary coil 33 as coil 31 moves past it in close, inductively coupled proximity. This causes a voltage to be induced across coil 33 and the resulting current in coil 33 will be sufficiently high and of proper polarity to turn on the SCR 38 and thereby energize the control relay R.

An essentially similar action takes place if, while the first coil 31 on the transfer cylinder is moving in close proximity past the first stationary coil 34 and the second transfer cylinder coil 32 is moving in close proximity past the second stationary coil 33, any one or several of the gripper fingers 27 come into direct contact with the gripper posts 29 because of the absence or mispositioning of the sheet whose leading end is supposed to be gripped between the gripper fingers 27 and the gripper posts 29. In that event, the current through coil 33 will turn on SCR 38 and cause the control relay R to be energized.

When relay R is energized, it operates any suitable trip mechanism (not shown) to put the press into a nonprinting condition, for example, a trip mechanism of the general type shown in the aforementioned US. Pat. No. 2,578,700. Also, switch S will be opened manually by the press operator to disconnect the present control from its power supply after the missing, torn, or folded sheet has been noted and cleared from the press. Until switch S is opened, SCR 38 will remain on and relay R will remain energized even after the voltage induced across coil 33 has disappeared.

If desired, switch S may be arranged to be opened automatically in response to the energization of relay R, instead of being opened manually by the press operator.

The opening of switch 5 deenergizes relay coil R and turns off the SCR 38, so that they will be in their normal nonconducting condition after switch 5 is reset to its normal closed condition.

Consequently, it is seen that each time gripper finger 27 electrically contacts post 29 (or each time gripper finger 28 electrically contacts post 30) a current-carrying series circuit is completed with movable coils 31 and 32. if, at this time, coil 32 is moving past coil 34 then current will flow through this circuit to energize coil 33. In a broad sense then, coil 33, together with its associated circuit components SCR 38 and relay R, serve as control means responsive to current flowing in this series circuit for providing an output indication that a sheet is absent from the sheet gripper means.

From the foregoing description it will be apparent that the present control is relatively simple, operating from a relatively low voltage DC power supply, and not requiring an oscillator or relatively complex circuitry to perform the desired control functions in a reliable manner. The present control has only two possible conditions at the coil 33 which controls the energization of the control relay R: either substantially no current is present or a tum-on current is present there. There is no possibility of a spurious signal which would turn on the control relay R when the coils on the transfer cylinder move away from the stationary coils, and therefore the control circuit need not be disabled during each half revolution of the transfer cylinder to prevent such a spurious response.

It is to be understood that the present control also is provided on the feed cylinder 20 for operation by the grippers 26 thereon to detect the absence of a sheet coming into the first printing unit A of the press in essentially the same manner as described with reference to the transfer cylinder 24, except that in the case of the feed cylinder 20 the sheet detection operation takes place only once for each rotation of this cylinder.

While a presently preferred embodiment of this invention has been described in detail with reference to the accompanying drawing, it is tobe understood that various modifications and adaptations which differ from the disclosed embodiment may be adopted without departing from the spirit and scope of the present invention. For example, the present control may be associated with any other cylinder of each printing unit, such as the impression cylinder 13, instead of with the transfer cylinder 24.

Having described my invention, 1 claim:

1. In a sheet-handling apparatus having a cyclically movable sheet-handling member, and sheet gripper means on said member, a control arrangement comprising: a stationary coil, DC power supply means connected to and continuously applying current to said stationary coil, a movable coil mounted on said sheet-handling member for movement in close proximity past said stationary coil during a predetermined portion of the cyclic movement of said sheet-handling member so as to cut the electromagnetic flux produced by said stationary coil, said sheet gripper means being operatively connected to said movable coil to complete a currentcarrying circuit therewith to cause a voltage to be induced across said movable coil and for current to flow through said current-carrying circuit as said movable coil moves past said stationary coil and cuts the latters flux when a sheet is absent from said gripper means,

and control means responsive to current flowing in said cur rent-carrying circuit for providing an output indication that a sheet is absent from said sheet gripper means. I

2. Apparatus according to claim 1, wherein said control means comprises an electrically energizable control element and normally open switch means connected across said DC power supply means, said switch means being operable as a result of the voltage induced across said movable coil and current flowing in said current-carrying circuit to complete the energization circuit for said control element from said DC power supply means.

3. Apparatus according to claim 2, wherein said switch means is a semiconductor switch device.

4. In a sheet-handling apparatus having a cyclically movable sheet-handling member, and sheet gripper means on 'said member comprising confronting sets of gripper members which grip a sheet when said gripper members are together, said gripper members being of electrically conductive material, a control arrangement comprising a spaced pair of stationary coils, means for energizing one of said stationary coils, a spaced pair of movable coils which are mounted for movement in unison with said sheet-handling member past said stationary coils to be in close proximity and inductively coupled respectively to said stationary coils simultaneously during a predetermined portion of the cyclic movement of said sheet handling member wherebythe movable coil which is passing in close proximity to said .one stationary coil cuts the electromagnetic flux produced by the latter, control means opera! tively connected to the other stationary coil to be operated in response to current through the latter, and said movable coils being connected to each other and to said gripper members such that when any of the'confronting sets of gripper members engage each other directly a voltage is induced in one movable coil as it moves past said one stationary coil and is applied across the other movable coil as it moves past said other stationary coil to produce a current in said other stationary coil for operating said control means.

5. Apparatus according to claim 4, wherein said means for energizing said one stationary coil is a DC voltage source.

6. Apparatus according to claim 5, wherein said control means operated by the current in said other stationary coil comprises an electrically energizable control element and a controlled rectifier connected in series with each other across said DC voltage source, and said other stationary coil is connected between the gate electrode of said controlled rectifier and one tenninal of said DC voltage source to apply a turn-on signal to said controlled rectifier in response to the voltage applied across said other movable coil.

7. Apparatus according to claim 5, wherein said movable coils are connected directly to each other at one end thereof, one of said movable coils has its opposite end grounded and the other of said movable coils has its opposite end connected directly to one gripper member of each set, and the other gripper member of each set is grounded.

8. Apparatus according to claim 7, wherein said control means comprises an electrically energizable control element and a controlled rectifier connected in series with each other across said DC voltage source, and said other stationary coil is connected between the gate electrode of said controlled rectifier and one terminal of said DC voltage source.

9. In a printing press having a rotary cylinder, a plurality of electrically conductive material and being electrically insulated from said cylinder, said gripper fingers being of electrically conductive material and being grounded to said cylinder, a control arrangement comprising: a spaced pair of stationary coils, a DC power supply connected to one of said stationary coils, a spaced pair of movable coils attached to said cylinder to revolve in unison therewith past said stationary coils, said cylinder in one portion of its rotation moving one of said movable coils in close proximit y past one of said stationary coils and at the same time moving the other of said movable coils in close proximity past the other of said stationary coils, said movable coils being directly connected to each other at one end thereof, one of said movable coils having its opposite end grounded to the cylinder and the other of said movable coils having. its opposite end connected to the gripper posts, and control meansoperable in response to current through saidother stationary coil.

10. Apparatus according to claim 9, wherein said control means comprises an electrically energizable control element and a controlled rectifier connected in series with each other across said DC power supply, and said other stationary coil is connected between the gate electrode of said controlled rectifier and one terminal of said DC voltage source to apply a turn-on signal to said controlled rectifier in response to said current. a

l 1. In a sheet-handling apparatus having a cyclically movable sheet-handling member, and sheet gripper means on said member, a control arrangement comprising a stationary coil, electrical power supply means connected to said stationary coil to energize the latter with an electrical current, a movable coil mounted on said sheet-handling member for movement in closetproximity'past said stationary coil during a predetermined portion of the cyclic movement of said handling member when a sheet is to be present in said gripper means, means for transmitting a signal from said sheet-handling member comprising a first electrical means on said sheet-handling member and a second electrical means mounted adjacent said sheet-handling member with said first and second electrical means being arranged to be opposite each other during said predetermined portion of said cyclic movement, said second electrical means being activated to a predetermined state by said first electrical means when the first electrical means is adjacent said second electrical means and electrically energized, means interconnecting said movable coil and said first electrical means and responsive to the absence of a sheet in said gripper means during said predetermined portion of said cyclic movement to electrically connect said first electrical means to said movable coil in a series current-carrying circuit to energize said first electrical means from said movable coil in the absence of a sheet in said gripper means during said predetermined portion of the cyclic movement, and control means responsive to said predetermined state of said second electrical means for providing an indication of the absence of a said sheet.

12. In a sheet handling apparatus as defined in claim 11 wherein said first and second electrical means are coils.

13. In an apparatus according to claim 11 wherein said control means comprises an electrically energizable control element and a controlled rectifier connected in series with each other and said second electrical means is connected to the gate electrode of said controlled rectifier to apply a turn-on signal to said controlled rectifier in response to the activation of said second electrical means to said predetermined state. 

1. In a sheet-handling apparatus having a cyclically movable sheet-handling member, and sheet gripper means on said member, a control arrangement comprising: a stationary coil, DC power supply means connected to and continuously applying current to said stationary coil, a movable coil mounted on said sheethandling member for movement in close proximity past said stationary coil during a predetermined portion of the cyclic movement of said sheet-handling member so as to cut the electromagnetic flux produced by said stationary coil, said sheet gripper means being operatively connected to said movable coil to complete a current-carrying circuit therewith to cause a voltage to be induced across said movable coil and for current to flow through said current-carrying circuit as said movable coil moves past said stationary coil and cuts the latter''s flux when a sheet is absent from said gripper means, and control means responsive to current flowing in said current-carrying circuit for providing an output indication that a sheet is absent from said sheet gripper means.
 2. Apparatus according to claim 1, wherein said control means comprises an electrically energizable control element and normally open switch means connected across said DC power supply means, said switch means being operable as a result of the voltage induced across said movable coil and current flowing in said current-carrying circuit to complete the energization circuit for said control element from said DC power supply means.
 3. Apparatus according to claim 2, wherein said switch means is a semiconductor switch device.
 4. In a sheet-handling apparatus having a cyclically movable sheet-handling member, and sheet gripper means on said member comprising confronting sets of gripper members which grip a sheet when said gripper members are together, said gripper members being of electrically conductive material, a control arrangement comprising a spaced pair of stationary coils, means for energizing one of said stationary coils, a spaced pair of movable coils which are mounted for movement in unison with said sheet-handling member past said stationary coils to be in close proximity and inductively coupled respectively to said stationary coils simultaneously during a predetermined portion of the cyclic movement of said sheet handling member whereby the movable coil which is passing in close proximity to said one stationary coil cuts the electromagnetic flux produced by the latter, control means operatively connected to the other stationary coil to be operated in response to current through the latter, and said movable coils being connected to each other and to said gripper members such that when any of the confronting sets of gripper members engage each other directly a voltage is induced in one movable coil as it moves pAst said one stationary coil and is applied across the other movable coil as it moves past said other stationary coil to produce a current in said other stationary coil for operating said control means.
 5. Apparatus according to claim 4, wherein said means for energizing said one stationary coil is a DC voltage source.
 6. Apparatus according to claim 5, wherein said control means operated by the current in said other stationary coil comprises an electrically energizable control element and a controlled rectifier connected in series with each other across said DC voltage source, and said other stationary coil is connected between the gate electrode of said controlled rectifier and one terminal of said DC voltage source to apply a turn-on signal to said controlled rectifier in response to the voltage applied across said other movable coil.
 7. Apparatus according to claim 5, wherein said movable coils are connected directly to each other at one end thereof, one of said movable coils has its opposite end grounded and the other of said movable coils has its opposite end connected directly to one gripper member of each set, and the other gripper member of each set is grounded.
 8. Apparatus according to claim 7, wherein said control means comprises an electrically energizable control element and a controlled rectifier connected in series with each other across said DC voltage source, and said other stationary coil is connected between the gate electrode of said controlled rectifier and one terminal of said DC voltage source.
 9. In a printing press having a rotary cylinder, a plurality of gripper posts on said cylinder, a plurality of gripper fingers carried by said cylinder and pivotally movable thereon toward and away from said gripper posts, said gripper posts being of electrically conductive material and being electrically insulated from said cylinder, said gripper fingers being of electrically conductive material and being grounded to said cylinder, a control arrangement comprising: a spaced pair of stationary coils, a DC power supply connected to one of said stationary coils, a spaced pair of movable coils attached to said cylinder to revolve in unison therewith past said stationary coils, said cylinder in one portion of its rotation moving one of said movable coils in close proximity past one of said stationary coils and at the same time moving the other of said movable coils in close proximity past the other of said stationary coils, said movable coils being directly connected to each other at one end thereof, one of said movable coils having its opposite end grounded to the cylinder and the other of said movable coils having its opposite end connected to the gripper posts, and control means operable in response to current through said other stationary coil.
 10. Apparatus according to claim 9, wherein said control means comprises an electrically energizable control element and a controlled rectifier connected in series with each other across said DC power supply, and said other stationary coil is connected between the gate electrode of said controlled rectifier and one terminal of said DC voltage source to apply a turn-on signal to said controlled rectifier in response to said current.
 11. In a sheet-handling apparatus having a cyclically movable sheet-handling member, and sheet gripper means on said member, a control arrangement comprising a stationary coil, electrical power supply means connected to said stationary coil to energize the latter with an electrical current, a movable coil mounted on said sheet-handling member for movement in close proximity past said stationary coil during a predetermined portion of the cyclic movement of said handling member when a sheet is to be present in said gripper means, means for transmitting a signal from said sheet-handling member comprising a first electrical means on said sheet-handling member and a second electrical means mounted adjacent said sheet-handling member with said firsT and second electrical means being arranged to be opposite each other during said predetermined portion of said cyclic movement, said second electrical means being activated to a predetermined state by said first electrical means when the first electrical means is adjacent said second electrical means and electrically energized, means interconnecting said movable coil and said first electrical means and responsive to the absence of a sheet in said gripper means during said predetermined portion of said cyclic movement to electrically connect said first electrical means to said movable coil in a series current-carrying circuit to energize said first electrical means from said movable coil in the absence of a sheet in said gripper means during said predetermined portion of the cyclic movement, and control means responsive to said predetermined state of said second electrical means for providing an indication of the absence of a said sheet.
 12. In a sheet handling apparatus as defined in claim 11 wherein said first and second electrical means are coils.
 13. In an apparatus according to claim 11 wherein said control means comprises an electrically energizable control element and a controlled rectifier connected in series with each other and said second electrical means is connected to the gate electrode of said controlled rectifier to apply a turn-on signal to said controlled rectifier in response to the activation of said second electrical means to said predetermined state. 